1. Field of the Invention
This invention generally relates to fluid filters for use in pressurized fluid systems and more particularly to apparatus for purging contaminants from and drying a desiccant material within a filter.
2. Description of the Prior Art
A conventional fluid filter for use in compressed air systems of the general type described herein is shown in U.S. Pat. No. 3,832,831, issued on Sept. 3, 1974. Such prior art air filters normally purge and reactivate a filter and desiccant medium contained therein by an unrestrained, variable rate, reverse air flow exhausting to the ambient atmosphere. In purging such conventional air filters, a drain valve is instantaneously unseated, resulting in a sudden decompression of the filtering unit. The pressurized air reacts to the sudden decompression of the filtering unit by flowing at a high volume and varying rate in a reverse direction through the filtering unit. The rapid unrestrained flow of decompressed air through the filter and desiccant bed tends to powder or pulverize and destroy the individual desiccant beads conventionally comprising the desiccant material, thus shortening its life. This, of course, results in the disadvantage of the compressed air system frequently being out of service for replacement of the desiccant bed.
Attempts have been made in the past to reduce the velocity at which decompressed air flows in reverse direction through the desiccant bed as by providing a metering valve in the system to control the volume of purging air permitted to flow through the filter, one such system being shown in U.S. Pat. No. 3,464,186, issued on Sept. 2, 1969. However, even though the volume of purging air is metered or restricted to the filter, its flow rate through the filter varies, which is detrimental to the life of the desiccant medium.
It has been discovered that when the reverse flow of air through the filter is controlled at a high volume constant velocity, its filter unit is more effectively cleaned and the life of the desiccant material is substantially prolonged.